Fixes a number of issue related to test portability on Windows.

99% of this CL is simply moving calls to "import pexpect" to a more
narrow scope - i.e. the function that actually runs a particular
test.  This way the test suite can run on Windows, which doesn't have
pexpect, and the individual tests that use pexpect can be disabled on
a platform-specific basis.

Additionally, this CL fixes a few other cases of non-portability.
Notably, using "ps" to get the command line, and os.uname() to
determine the architecture don't work on Windows.  Finally, this
also adds a stubbed out builder_win32 module.

The full test suite runs correctly on Windows after this CL, although
there is still some work remaining on the C++ side to fix one-shot
script commands from LLDB (e.g. script print "foo"), which currently
deadlock.

Reviewed by: Todd Fiala

Differential Revision: http://reviews.llvm.org/D4573